Both tamoxifen and Evista (chemical name: raloxifene) have been shown to lower the risk of breast cancer in women at high risk for breast cancer. The medicines also reduce the risk of hormone-receptor-positive breast cancer coming back (recurrence).

A new study underscores the effectiveness of these medicines, but also found that they aren’t widely prescribed by doctors or taken by women at high risk of breast cancer because of concerns about side effects.

Both tamoxifen and Evista are hormonal therapy medicines called SERMs (selective estrogen receptor modulators). SERMs block the action of estrogen in breast and certain other cells by sitting in the cells’ estrogen receptors. SERMs don’t affect all estrogen receptors the same way because they’re selective (as the name says). In bone cells, SERMs interact with the receptors the way estrogen does and strengthen bones. In breast cells, SERMs block the receptors’ interaction with estrogen and limit cell growth.

Earlier studies have shown that SERMs reduce the risk of breast cancer in high-risk women, but researchers don’t know exactly how long this reduction in risk lasts. So the researchers for this study decided to do a meta-analysis – a study that combines and analyzes the results of earlier studies – to determine how effective SERMs were at reducing breast cancer risk.

Besides tamoxifen and Evista, some of the studies the researchers reviewed included two other SERMs: lasofoxifene and arzoxifene. Both medicines are under development and neither is approved yet by the U.S. Food and Drug Administration to be used to reduce breast cancer risk.

The researchers looked at nine studies involving more than 83,000 women. About half of the studies had a follow-up time of more than 5.5 years and half of the studies had shorter follow-up times. In most of the studies, the women took a SERM for 5 years.

Overall, the researchers found that taking a SERM reduced breast cancer risk by about 38% 10 years after the women started taking the medicine:

6.3% of the women who didn’t take a SERM were diagnosed with breast cancer

4.2% of the women taking a SERM were diagnosed with breast cancer

Taking a SERM offered more benefits in the first 5 years than it did in the second 5 years.

Tamoxifen seemed to be a little better at reducing breast cancer risk than Evista.

Still, tamoxifen may cause side effects, some of them serious, including:

blood clots

stroke

a higher risk of endometrial cancer

cataracts

leg cramps

weight gain

sweating

hot flashes

joint pain

While Evista also may cause blood clots and stroke, women taking Evista are less likely to have these two serious side effects compared to women taking tamoxifen. Evista also may cause:

hot flashes

sweating

joint pain

weight gain

These side effects are the reasons many women decide not to take a SERM to reduce breast cancer risk.

The researchers who did this meta-analysis said that more research is needed to figure out the long-term risk-reduction effects of SERMs, as well as the women who would benefit most from taking a SERM. Many of the studies they reviewed ended before the women had finished taking a SERM for 5 years. So there was no information on the women’s breast cancer risk at 15 or 20 years after they took a SERM.

Right now, most doctors use some form of the Gail model, a standard breast cancer risk assessment tool. The Gail model assesses breast cancer risk based on a series of personal health questions that women and their doctors answer together. The questions ask about risk factors such as age, child-bearing history, family history of breast cancer, and breast biopsy results. The result is a Gail score, which estimates the risk of developing invasive breast cancer in the next 5 years. Some more recent versions of the Gail model also include alcohol use, menopausal status, and body mass index. Some doctors wonder if genetic testing or other information should be added to the Gail model. More research needs to be done to figure out the most accurate way to assess a woman’s risk of breast cancer.

If you have a higher-than-average risk of breast cancer, you already might be taking medicine to help keep your risk as low as it can be. If not, you may want to ask your doctor if taking a SERM – tamoxifen or Evista – makes sense for you. Besides lowering your breast cancer risk, each medicine has benefits and side effects. If taking a SERM makes sense for you, talk to your doctor about any side effects and how to manage them. Together, you can make the best choice for your unique situation.

You can learn more about tamoxifen and Evista in the SERMs pages of the Breastcancer.org Hormonal Therapy section.

Editor's Note: In July 2013, the American Society of Clinical Oncology put out new guidelines on using hormonal therapy medicines to reduce breast cancer risk in high-risk undiagnosed women. Besides tamoxifen and Evista, the guidelines also recommend that doctors talk to postmenopausal high-risk women about using the aromatase inhibitor Aromasin (chemical name: exemestane) to reduce risk.

Research presented at the 2013 San Antonio Breast Cancer Symposium showed that Arimidex (chemical name: anastrozole) can lower the risk of first-time, hormone-receptor-positive breast cancer in postmenopausal women at high risk who haven’t been diagnosed. Arimidex isn’t approved by the FDA for this use, but doctors may consider it a good alternative to other hormonal therapies approved to reduce risk in high-risk women.